The present invention relates to a memory device and, more particularly, to a static memory device employing insulated gate field effect transistors (IGFET).
In the static memory device, write operation to and read operation from memory cells of the memory device are performed through a pair of digit lines provided with each unit of column, as is well known to a person skilled in the art. The pair of digit lines are connected to a constant voltage source through load elements, for example, resistors. In this type of memory device it occurs that by applying logic data, e.g. logical "0", to the digit line pair, the data is written into one memory cell coupled with the digit line pair and thereafter a stored information in another memory cell also coupled with the same digit line pair is read-out. In such a case, after the write operation and prior to the read operation, it is necessary to remove a potential level difference between the digit line pair which has resulted from the write operation. If this is not done, the worst case possibly occurs in that the level difference left in the digit lines is erroneously written into the memory cell selected to be subjected to the read operation. This leads to an instability in the memory operation. The erroneous operation is problematic particularly in the case where the read data is an inverse data, e.g. logical "1". Therefore, the read operation undertaken after the write operation must be performed after the digit line pair is balanced in potential level. Also in this case, when the digit line pair having a write level of one logical data responds to a read level of another logical data, it experiences a large level change, leading to a delay or an elongation of the access time. The problem may be solved by employing load elements having a large conductance for the digit line pair to shorten the time required for balancing the digit line pair. This approach, however, requires an enlargement of the shapes of the transistors of the flip-flop circuits and their peripheral circuits in order to increase the driving capability of the transistors. Accordingly, it is difficult to obtain a high density memory device and besides, the current flowing through the digit lines increases, so that the power consumption increases.